1. Field of the Invention
The present invention relates to a transmission for coupling a starter-generator electrical machine to an internal combustion engine in a way to assure satisfactory functional reliability of transmission of torque even when the electrical power fails.
2. Description of the Related Art
In modern motor vehicles, increasing use is being made of electrical machines, which are usable both for starting the internal combustion engine and as generators for charging the battery during certain operating conditions. The starter function can be used additionally to supply additional starting torque in certain operating conditions. With the generator function it is possible to recover energy when braking the vehicle, for example, or to recharge the battery during certain operating conditions. Thus the electrical machine serves both to improve driving convenience and to reduce fuel consumption.
For coupling between the electrical machine and the internal combustion engine, a transmission is provided that operates at a high transmission ratio between the rotational speed of the electrical machine and the rotational speed of the internal combustion engine when the internal combustion engine is running at low rotational speeds, and with a low transmission ratio when the internal combustion engine is running at high rotational speeds. The result is that the electrical machine supplies sufficient electrical energy at low rotational speeds of the internal combustion engine and that sufficient torque is available for starting the internal combustion engine. The lower transmission ratio ensures that the electrical machine does not overspeed at high rotational speeds of the internal combustion engine.
A transmission ratio control device is normally actuated by centrifugal force, so that the system operates with a high transmission ratio at low rotational speeds. With a good battery charge even the low transmission ratio is sufficient, since the battery does not have to be charged and lower driving losses occur for the electrical machine. It therefore makes sense with regard to energy to actively switch over the transmission ratio in accordance with need.
Active switching results in a conflict of objectives for functional reliability when the electrical power fails. If the control system operates so that a low transmission ratio continues in effect when the electrical power fails, it can no longer be possible to start the internal combustion engine, since there is not sufficient torque available.
If the control is instead operated so that a high transmission ratio remains in effect when the electrical power fails, the electrical machine overspeeds at high crankshaft rotational speeds and is destroyed.
An object of the present invention is to further develop an appropriate type of transmission such that satisfactory functional reliability is ensured even when the electrical power fails.